The Nature of the Binding of Inhibitors to Pepsin and the Kinetics of Inhibited Peptic Hydrolysis of N-Acetyl-L-phenylalanyl-L-tyrosine

Inhibition of pepsin by polyions and c.d. studies

The effects of poly(vinyl sulphate) (PVS), poly(vinyl pyrrolidone) (PVP) and protamine sulphate with the enzyme pepsin (EC 3.4.23.1) have been investigated. PVS, PVP and protamine acted as inhibitors of the enzyme pepsin at low concentrations, but at high concentrations of the polyions (with the exception of PVS) the inhibition was less pronounced. The catalytic effectiveness of several polyions has been shown experimentally; when used at pH 2.1 with haemoglobin and at pH 5.0 with azocasein thus acted as a weak proteolytic catalyst. The order of effectiveness was: polybrene greater than poly(L-lysine) (PLL) greater than spermine greater than spermidine greater than protamine greater than and PVP relative to pepsin. The hydrolysis of the substrates by the enzyme decreases in the presence of high concentrations of monovalent and/or divalent salts. The purity of the enzyme was assessed by determination of the molecular weight using sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), phosphorus and nitrogen content. The circular dichroism (c.d.) spectrum of the enzyme in the wavelength range 240-310 nm, where the polycations did not have a c.d. spectrum has also been studied. Each of the polyions had a definite effect on the c.d. spectrum, showing strong binding to the enzyme. The neutral polymer PVP was also found to modify the c.d. spectrum, showing strong binding to the enzyme. The strengths of the interactions as indicated by the magnitudes of the changes in the c.d. spectrum of pepsin at 270 nm were: polybrene greater than protamine greater than PLL greater than PVP greater than spermidine greater than spermine.

The effect of polycations on the activity of pepsin

A study has been made of the interaction of selected polycations with pepsin A (E.C. 3.4.23.1). Protamine, polybrene, spermine, spermidine and poly(L-lysine) all acted as inhibitors of the enzyme at low concentrations, but at higher concentrations of the polycations the inhibition was less pronounced. A more detailed study of the anomalous inhibition was made using protamine, polybrene and poly(L-lysine) and it was shown experimentally that, when used by themselves, each of these polycations acted as a weak proteolytic catalyst. The order of catalytic effectiveness was: protamine greater than polybrene much greater than poly(L-lysine). Thus, it is now possible to explain why the observed inhibition of pepsin decreases when the concentration of the inhibiting polycation is increased.

Inhibition of peptic activity by carbenoxolone and glycyrrhetinic acid

Carbenoxolone (Biogastrone, Berk) has been shown to reduce the peptic activity and total acidity of gastric juice obtained from anaesthetized pylorus-ligated rats without affecting significantly the volume of gastric juice secreted or the K(+) concentration. Glycyrrhetinic acid was less potent in reducing peptic activity and caused no reduction in total acidity. Antipeptic activity of carbenoxolone has also been demonstrated in vitro using the pepsin plate technique and the haemoglobin pepsin assay.It is suggested that these actions of carbenoxolone may contribute to the increased rate of healing of peptic ulcer in patients treated with the drug.